The present invention relates to an apparatus for measuring stress, especially relates to an apparatus for measuring the unavoidably generated stress in a thin film in an ultra high sensitivity and on a real time and quantitative basis. The present invention also relates to a manufacturing method for a probe which is used for the aforementioned measuring apparatus, especially to a manufacturing method for a probe by an optimal arrangement of the optical fiber strands for introducing light and the receiving optical fiber strands.
Recently, most of the high technology devices are manufactured as a thin film type, but the thin film always has an unavoidable stress during the film formation. The stress inevitably changes various physical properties of the thin film. Accordingly, the stress may influence the quality of the device manufactured in a thin film type and may further lead to lower productivity. Therefore, in order to manufacture high quality devices, studies on reducing stress in a thin film are under progress.
Furthermore, development of stress measuring apparatus which measures stress not only during the thin film deposition on a real time basis but also during the process of atomic layer deposition. This is because the accurate measurement of stress not only is an important tool in the studies of the structural characteristics of the thin film but also plays an important role in controlling the degree of deformation.
Generally, stress has a close relationship with the thin film growth mechanism, developed microstructure, deposition condition of a thin film. In addition, the stress is a combined result of many factors such as a thermal stress resulting from the difference in the thermal expansion coefficient between the substrate and thin film, stress from the lattice mismatch, and inherent stress related to the microstructure of the thin film.
In the prior art methods, to measure these stresses, either a method utilizing X-ray or a method measuring a bending of a substrate has been used.
The method utilizing X-ray is a fundamental stress measuring method, but this method is utilizing a lattice diffraction which is not applicable to all types of thin films. Furthermore, even though the sample has a good lattice, the stress generated during the thin film formation is difficult to quantify on a real time basis.
The bending of a substrate can be measured by several methods which include a method utilizing the variation of a capacitance, a laser scanning method, or a Y-type non-contact displacement measurement method utilizing an optical fiber.
The capacitance measuring method measures the variation of a capacitance between a substrate and an electrode by the bending of a substrate during a thin film deposition, which is not applicable to a sample manufactured by a sputtering method due to a plasma.
The laser scanning method utilizes a position sensitive detection photodiode to measure the variation of the reflection angle of a laser beam according to the bending of a substrate after the beam is incident on the back side of the substrate. This method is mainly used for the circular samples.
In the Y-type non-contact displacement measurement method reported by H. Akimoto et al., a fiber optic bundle in the vicinity of a substrate directs a light beam toward the back side of the substrate and the bending of the substrate is measured from the intensity variation of a reflected beam. The sensitivity of a stress measuring apparatus used by Akimoto was 0.117 xcexcm/mV.
The stress "sgr"f on a thin film can be determined from the bending of a thin film according to the following Stoney Formula.       σ    f    =            δ      ⁢              xe2x80x83            ⁢              E        s            ⁢              d        s        2                    3      ⁢              xe2x80x83            ⁢                        l          2                ⁡                  (                      1            -                          v              s                                )                    ⁢              xe2x80x83            ⁢              d        f            
Where l is the width of the substrate, df is the thickness of the thin film, ds is the thickness of the substrate, vs is the Poisson""s ratio, Es is the Young""s modulus, xcex4 is the degree of bending of a substrate.
This formula is obtained from a equilibrium condition of a bending moments and a static equilibrium condition of forces generated between each interface. This equation reveals that to measure the accurate real time stress, bending of a substrate according to the film thickness should be measured precisely. Increase of xcex4 during the increase of thickness of a thin film indicates tensile stress and decrease of xcex4 indicates compressive stress.
As described above, methods and apparatuses measuring the degree of stress have been introduced by previous arts, but they exhibit low measurement reliabilities due to the difficulties in the stress measurement and the low sensitivity. Therefore, it is difficult to form a high quality thin film on a substrate due to the difficulties in the process control of the thin film deposition.
Accordingly, it is an object of the present invention to provide an apparatus for measuring stress in a thin film which has an atomic layer level resolution to manufacture high quality devices.
Another object of the present invention is to provide a method of manufacturing a fiber optic bundle probe used for the above stress measuring apparatus.
In order to achieve the aforementioned object, the stress measuring apparatus of the present invention comprises a fiber optic bundle probe including:
a plurality of optical fiber strands for introducing light to the back side of a substrate on which a thin film is formed;
a plurality of optical fiber strands placed symmetrically around each of the introducing optical fiber strands in order to receive light reflected from the back side of the substrate; and
a capillary tube for integrating the introducing and receiving optical fiber strands by inserting parts of them therein.
Only one side end of the substrate is fixed on a substrate holder to allow a substrate bending during a thin film formation. The apparatus further comprises a distance control means which controls the distance between the back side of a substrate and a probe. There also is a photodetecting means which converts light signals from the receiving optical fiber to electrical signals.
In the present invention, the outer side of each introducing and receiving optical fiber strands is preferably coated with epoxy to prevent the damage from a sheer force. Also, light from a halogen lamp can be utilized as the light directed to the introducing optical fiber strands. A translator with a differential micrometer which can make a fine control of a distance can be used. More preferably, the apparatus further comprises means which amplifies the electrical signals and receives only direct current signals therefrom to eliminate noises. Still furthermore, for the application of the apparatus to thin films formed within a vacuum chamber, the apparatus may further comprise means for inserting the introducing and receiving optical fiber strands within the vacuum chamber while maintaining the air-tightness of the chamber.
In order to achieve the another object, the method of manufacturing a probe comprises the steps of:
preparing a plurality of optical fiber strands for introducing light to the back side of a substrate on which a thin film is formed and a plurality of optical fiber strands for receiving light reflected from the back side of the substrate;
coating the outer side of each introducing and receiving optical fiber strands with epoxy;
integrating the introducing and receiving optical fiber strands by inserting parts of the strands within a capillary tube; and
polishing the ends of the introducing and receiving optical fiber strands.
In the above manufacturing method, the integrating step of the optical fiber strands preferably comprises the steps of:
varying the arrangement of the introducing and receiving optical fiber strands in a two dimension random structure;
introducing light to the introducing optical fiber strands and then reflecting the light at the back side of the substrate;
receiving the reflected light by the receiving optical fiber strands;
detecting the received light; and
determining the positions of the introducing and receiving optical fiber strands, which maximizes the intensity variation of the detected light according to the distance variation between the back side of the substrate and said probe.